Selective bridge amplifiers



June 1959 A. A. HEMPHILL ET AL 2,890,290

SELECTIVE BRIDGE AMPLIFIERS Filed May '7, 1954 INVENTORS' ALFRED A.-HEMPHILL JOHN M-TEWKSB URY A'TTORNE YS United States Patent SELECTIVE BRIDGE AMPLIFIERS Alfred A. Hemphill, Baltimore, and John Merle Tewltsbury, Lutherv-ille, Md assignors to Bendix Aviation Corporation, T'owson, Md., a corporation of Delaware Application May 7', 1954, Serial No. 428,310

6 Claims. (Cl. 179-171) A This invention relates to selective amplifiers, and in particular, to selective bridge amplifiers.

It is often desirable to provide in a system a signal selective device of such a character as to decrease the effect of the undesired signals while maintaining the effect of the desired ones. The present invention provides a means for accomplishing this result.

This invention utilizes the characteristics of a cathode follower, a bridge circuit and a differential amplifier in a circuit so arranged that desired signals are maintained While the undesired signals are greatly attenuated. The portion of the circuit that functions as a cathode follower couples the incoming signals to the portion of the circuit providing the bridge circuit characteristics. The output of the bridge portion is applied to the portion of the circuit functioning as a differential amplifier. The output of the differential amplifier portion is a function of the signal measured between the output terminals of the bridge portion. The bridge portion is so designed that the ratio of the desired to undesired signals measured between the output terminals of the bridge portion is greater than the ratio between the desired and undesired signals in the input signals.

An object of this invention is to provide a means capable ofproducing a high degree of selectivity between signals of different frequencies.

Another object of this invention is to provide an efficient means for producing a high degree of signal selectivity at intermediate frequencies.

A further object of this invention is to provide a means capable of producing a high degree of signal selectivity when utilizing low impedance components.

A still further object of this invention is to provide a means capable of producing a high degree of signal selectivity which does not necessitate the use of high Q components.

Still another object of this invention is to provide a means, capable of producing a high degree of signal selectivity, that is relatively easy to adjust.

Referring to thedrawings:

Fig. 1 is a schematic diagram illustrating the invention;

Fig. 2 is a schematic diagram depicting a particular embodiment of the invention as illustrated in Fig. 1;

Fig. 3 is another schematic diagram illustrating the invention; and

Fig. 4 is a schematic diagram depicting a particular embodiment of the invention as illustrated in Fig. 3.

Referring in particular to Fig. l, a schematic diagram illustrating the invention is depicted. An input terminal 6, a coupling capacitor C a tube 1, a resistor R and a DC. voltage supply 13-}- constitute a cathode follower circuit. A bridge circuit, constructed of components Z Z Z and Z is connected across the output of the cathode follower. A tube 2, the supply 13+, a resistor R and, the components 2;, and Z constitute a differential amplifier. The components Z and Z perform dual functions. The output voltagesv produced by the bridge ice action are coupled into the differential amplifier by the connections of the points 3 and 4 of the bridge circuit to the cathode and grid, respectively, of the tube 2. A capacitor C is provided for coupling the output of the differential amplifier to an output terminal 7.

Fig. 2 illustrates a schematic diagram of a particular embodiment of the invention as shown in Fig. 1. Components in this diagram which have the exact form as in the diagram of Fig. l have been assigned the same identification markings. The component Z of Fig. 1 is made up of the inductors L and L and capacitors C and C while the components Z Z and Z; are now resistors R R and R respectively.

The resistors R and R must be. of such values that the proper operation of the differential amplifier is obtained. The components L L C C and R must be chosen to function with the resistors R and R to provide the desired output from the bridge circuit arrangement. With the particular arrangement shown in Fig. 2, it is wished to obtain one desired signal and to greatly attenuate two undesired signals. As the differential amplifier amplifies the signals appearing between the points 3 and 4, it is necessary to have balanced conditions at the frequencies of the undesired signals and a maximum unbalanced condition at the frequency of the desired signal. This is accomplished by selecting the values of the components L L C C and R to function with the components R and R; such that:

(1) At the frequency of one of the undesired signals, the series combination of L --C will resonate, and at the frequency of the other undesired signal, the series combination of L -C willresonate. Under these conditions, the impedance of Z will be sufiiciently resistive such that the degree of phase shift caused by any reactive characteristic will be negligible. Therefore efiicient reduction of the undesired signals is obtained if the value of the resistor R is so chosen that the signals appearing between the points 3 and 4- are at a minimum value at these frequencies.

(2) At the frequency of the desired signal, the parallel combination of the components L C L and C will resonate. Therefore the impedance of Z will be resistive' and of a large magnitude. With the value of the resistor R chosen in (l), a maximum signal difference will occur between the points 3 and 4.

For signals at frequencies other than, the frequencies of the desired and undesired signals, the impedance of Z will have a reactive component. This will. cause a phase shift to occur in the signals. appearing at the point 3, and therefore a distortion in the signals appearing between the points 3 and 4. This is not objectionable as these signals are not the ones to be used.

To provide additional attenuation of the undesired signals, a tuned circuit Z may be placed between the cathode of tube 1 and the ungrounded input terminal of the bridge circuit portion. This is illustrated in Fig. 3. This circuit is identical to Fig. 1 except for the addition of the tuned circuit Z Fig. 4 illustrates a particular version. of the tuned circuit Z of Fig. 3. This particular version is a series combination of a capacitor C and. an inductor L The insertion of the inductor L and the capacitor C provides additional attenuation of the undesired signals. This is accomplished by selecting these components so that'they will series-resonate at the frequency of the desired signal; Therefore, the series combination will present a low impedance at the frequency of the desired signal and a higher impedance at the frequencies of the undesiredv signals. As this combination is in seriesconnection with the. bridge. portion of the circuit, a voltage divider action takes. place. Consequently, the

ratio of the signals appearing across the bridge portion to the signals at the output of the cathode follower portion will be smaller at the undesired signals than at the desired signals.

An application of the circuit illustrated in Fig. 4 employed the following values of components for a desired signal the frequency of which was 142.5 kc. and undesired signals the frequencies of which were 140 and 145 kc.:

Several of the benefits derived from the application of the invention may be appreciated when considering the description presented in conjunction with Figs. 2 and 4. Because the invention incorporates the use of the cathode circuits of several vacuum tubes, the impedances of the various components may be relatively low in values. The Qs of the individual components are not critical as the circuit obtains its sharpness from the bridge action. These are desirable features when compact, inexpensive arrangements are desired.

Although a particular application of the invention has been depicted in Figs. 2 and 4, it is to be understood that this is not meant to limit the scope of the invention. Various other combinations of elements are possible to form the components Z Z Z and Z such as to produce other desirable outputs.

What is claimed is:

1. A selective bridge amplifier, comprising: a first electron tube connected as a cathode follower circuit including a load impedance element connected to the cathode of said tube; a bridge circuit comprising four serially connected arms, each of said arms including an impedance element, the impedance element in a first of said arms consisting of a pair of parallel branches, each of said branches consisting of a series resonant network tuned to the frequency of a respective undesired signal, the parallel combination of said branches being tuned to the frequency of a desired signal; means connecting the terminals of said load impedance element to opposite terminals of said bridge; a second electron tube having at least a plate, a cathode and a control grid; and means connecting said cathode and control grid of said second tube each to a respective one of the remaining two terminals of said bridge circuit.

2. A selective bridge amplifier, comprising: a first electron tube connected as a cathode follower circuit; a bridge circuit comprising four serially connected arms, each of said arms including an impedance element, the impedanee element in a first of said arms consisting of a pair of parallel branches, each of said branches consisting of a series resonant network tuned to the frequency of a respective undesired signal, the parallel combination of said branches being tuned to the frequency of a desired signal; means connecting the output of said cathode follower circuit across the junction of said first arm with a second of said arms and the opposite terminal of said bridge circuit; a second electron tube having at least a plate, a cathode and a control grid; means connecting said control grid to the junction of said first arm with a third of said arms and means connecting said cathode to the remaining terminal of said bridge circuit.

3. A selective bridge amplifier, comprising: a first electron tube having at least a plate, a cathode and a control grid; means connecting said electron tube as a cathode follower circuit including a load resistor connected between said cathode and a ground reference; a bridge circuit comprising four serially connected arms, each of said arms including an impedance element, the impedance element in a first of said arms consisting of a pair of parallel branches, each of said branches consisting of a series resonant network tuned to the frequency of a respective undesired signal, the parallel combination of said branches being tuned to the frequency of a desired signal; means connecting the junction of said cathode and said load resistor to the junction of said first arm with a second of said arms; a second electron tube having at least a plate, a cathode and a control grid; means connecting said control grid of said second tube to the junction of said first arm and a third of said arms; means connecting said cathode of said second tube to the junction of said second arm with the fourth of said arms and means connecting the remaining terminal of said load resistor and the junction of said third and fourth arms to said ground reference.

4. A selective bridge amplifier, comprising: a first electron tube connected as a cathode follower circuit including a load impedance element; a bridge circuit comprising four serially connected arms, each of said arms including an impedance element, the impedance element in a first of said arms consisting of a pair of parallel branches, each of said branches consisting of a series resonant network tuned to the frequency of a respective undesired signal, the parallel combination of said branches being tuned to the frequency of a desired signal; means including a series resonant network connecting said load impedance element across a pair of opposite terminals of said bridge, the last named net work being tuned to said desired signal; a second electron tube having at least a plate, a cathode and a control grid, and means connecting said cathode and connol grid of said second tube each to a respective one of the remaining two terminals of said bridge circuit.

5. A selective bridge amplifier, comprising: a first electron tube connected as a cathode follower circuit; a bridge circuit comprising four serially connected arms, each of said arms including an impedance element, the impedance element in a first of said arms consisting of a pair of parallel branches, each of said branches consisting of a series resonant network tuned to the frequency of a respective undesired signal, the parallel combination of said branches being tuned to the frequency of a desired signal, means connecting the output of said cathode follower circuit across the junction of said first arm with a second of said arms and the opposite terminal of said bridge circuit, a second electron tube having at least a plate, a cathode and a control grid, means connecting said control grid to the junction of said first arm with a third of said arms and means connecting said cathode to the remaining terminal of said bridge circuit, the said impedance element in said second arm being a resistor having a value of resistance such that the signal voltages appearing at said control grid and cathode of said second tube have a minimum value at the frequencies of said undesired signals.

6. A selective bridge amplifier, comprising: a first electron tube having at least a plate, a cathode and a control grid; means connecting said electron tube as a cathode follower circuit including a load resistor connected between said cathode and a ground reference; a bridge circuit comprising four serially connected arms, each of said arms including an impedance element, the impedance element in a first of said arms consisting of a pair of parallel branches, each of said branches consisting of a series resonant network tuned to the frequency of a respective undesired signal, the parallel combination of said branches being tuned to the frequency of 5 a desired signal; means including a series resonant network connecting the junction of said cathode and said load resistor to the junction of said first arm with a second of said arms, the last named network being tuned to said desired signal; a second electron tube having at least a plate, a cathode and a control grid; means connecting said control grid of said second tube to the junction of said first arm and a third of said arms; means connecting said cathode of said second tube to the junction of said second arm with the fourth of said arms and means connecting the remaining terminal of said load resistor and the junction of said third and fourth arms to said ground reference, the said impedance element in said second arm being a resistor having a value of resistance such that the signal voltages appearing at said control grid and cathode of said second tube have a minimum value at the frequencies of said undesired signals.

References Cited in the file of this patent UNITED STATES PATENTS Hallmark Oct. 28, 1947 2,535,912 Frank et al. Dec. 26, 1950 2,589,184 Zinn Mar. 11, 1952 

